It seems you have swelling of the gums in that region in relation to a tooth. This could be due to many reasons- a wisdom tooth erupting or a cavity in one of the last teeth causing a periapical abscess which may cause symptoms like these.
Next Steps
Please visit a dentist to check which tooth is the cause and treat it accordingly.
Health Tips
Hot water gargles with salt will help. Do NOT do any heat or cold application from outside.